With recent requirements for reduction in size and weight of electronic appliances, electric and electronic components are more and more reduced in size and weight. The electric and electronic components are exemplified by connectors, terminals, switches, relays, and lead frames.
For the reduction in size and weight of electric and electronic components, copper alloy materials for use in the components are designed to have more and more reduced thickness and width. Especially for integrated circuit (IC) use, copper alloy sheets having small thickness of from 0.1 to 0.15 mm have also been employed. As a result, copper alloy materials for use in such electric and electronic components should have much higher tensile strengths. For example, high-strength copper alloy sheets having a yield strength of 650 MPa or more are required typically in automobile connectors.
The copper alloy sheets for use in the components such as connectors, terminals, switches, relays, and lead frames should have not only high strengths and high electrical conductivity as mentioned above, but also satisfactory bending workability in severe bending such as U-bending (180-degree tight bending) in more and more cases.
In addition, the reduction in thickness and width of electric and electronic components causes reduction in cross-sectional area of electroconductive portions of copper alloy materials. The cross-sectional area reduction results in electroconductivity reduction. To supplement the electroconductivity reduction, the copper alloy materials themselves should have a satisfactory electrical conductivity of 30% IACS (International Annealed Copper Standard) or more.
For these reasons, a Corson alloy (Cu—Ni—Si copper alloy) has been used for electric and electronic components, because this alloy excels in the various properties as mentioned above and is inexpensive. In the Corson alloy, the solid solubility limit of a nickel silicate compound (Ni2Si) with respect to copper significantly varies with temperature. This alloy is a kind of precipitation-hardening alloys that are hardened by quenching/tempering. The Corson alloy has heat resistance and high-temperature strength at satisfactory levels and has been widely used typically in electroconducting springs and electric wires or cables for high tensile strength use.
However, the Corson alloy features significant strength anisotropy between a longitudinal direction (LD) (a direction parallel with the rolling direction) and a transverse direction (TD) (a direction perpendicular to the rolling direction), namely, has a strength in the transverse direction relatively lower than that in the longitudinal direction. The Corson alloy also features a large difference between its tensile strength (TS) and 0.2% yield strength (YP). For these reasons, the Corson alloy, when used in terminals/connectors, disadvantageously suffers typically from a low yield strength and an insufficient contact pressure strength in the transverse direction.
Independently, the Corson alloy, when designed to have a higher strength so as to have a higher contact pressure strength, disadvantageously suffers from cracking upon bending. Under such circumstances, demands have been made to develop a novel Corson alloy that has low strength anisotropy and excellent bending workability, which two properties are considered to be mutually contradictory.
To improve the bending workability of the Corson alloy, there have been proposed various techniques. Typically, Patent Literature (PTL) 1 proposes a technique of allowing a Corson alloy to contain Mg in addition to Ni and Si and controlling the S (sulfur) content of the alloy so as to improve strength, electroconductivity, bending workability, stress relaxation resistance, and coated-layer adhesiveness to suitable levels. PTL 2 proposes a technique of subjecting a Corson alloy to a solution heat treatment and then to aging without cold rolling so as to control inclusions to have sizes of 2 μm or less and to control the total amount of inclusions each having a size of from 0.1 μm to 2 μm to 0.5% or less of the total volume.
In addition, techniques of controlling grain textures are proposed so as to allow a Corson alloy to have better bending workability. Typically, PTL 3 proposes a copper alloy sheet made from a Corson alloy containing Ni in a content of from 2.0 to 6.0% in mass percent and Si in a mass ratio of Ni to Si of from 4 to 5. This Corson alloy is controlled to have an average grain size of 10 μm or less and to have a texture including cube orientation {001}<100> in a percentage of 50% or more as measured by SEM-EBSP analysis and has no lamellar boundary observable by microstructure observation with an optical microscope at 300-fold magnification.
Above-mentioned PTL 3 discloses a sheet of a Corson alloy, in which the Corson alloy has an electrical conductivity of from about 20% to about 45% IACS, has a high strength in terms of tensile strength of from about 700 to about 1050 MPa, and exhibits satisfactory bending workability. This copper alloy sheet is obtained in a manner as follows. When a copper alloy rolled sheet of a Cu—Ni—Si copper alloy (Corson alloy) is subjected to finish cold rolling, the sheet is cold-rolled to a working ratio of 95% or more before a final solution treatment, subjected to the final treatment, further cold-rolled to a working ratio of 20% or less after the final solution treatment, and subjected to aging so as to control the Corson alloy to have the microstructure as mentioned above.
PTL 4 discloses a technique of controlling a Cu—Ni—Si copper alloy to have such diffraction intensities of {420} plane and {220} plane as to satisfy conditions expressed as follows: I{420}/I0{420}>1.0, I{220}/I0{220}≦3.0 and thereby allowing the alloy to have better bending workability.
Independently, PTL 5 proposes a technique of increasing the amount of solutes after solution heat treatment so as to eliminate or mitigate strength anisotropy.
PTL 6 proposes a technique of controlling grain shape so as to eliminate or mitigate strength anisotropy. This technique reduces strength anisotropy by performing rolling to a final rolling reduction of 3.0% or less and thereby allowing grains to have reduced length both in the longitudinal direction and in the transverse direction.
PTL 7 proposes a technique of controlling the diffraction intensity of {220} crystal plane and the diffraction intensity of {200} crystal plane respectively so as to provide low strength anisotropy and better bending workability.